Coiled tubing has proven its value and utility in a variety of applications, although a number of technological challenges remain. One such technological challenge involves the need for a technique for the injection of wire, cable, or small flexible tube into a reel of coiled tubing. As used herein, the term "cable" refers to all manner or wire or cable, now known or later developed, for the conduction of electromagnetic energy. The present invention is particularly adaptable but not limited to wireline applications.
In the past, cable has been installed on coiled tubing by first unreeling the coiled tubing from the reel and installing cable into the straightened tubing. There are two methods currently used for installing wire or cable into coiled tubing in this way. One method involves hanging the tubing from the reel into a well, dropping the cable into the tubing by gravity feed, and reeling in the tubing with the cable inside. The other method involves taking the coiled tubing to a long, unobstructed stretch of ground and stretching out 10,000-20,000 feet of coiled tubing. Then, a pig attached to the cable is inserted into the end of the tubing and is pumped through the tubing. Each of these methods costs about $20,000 to perform including the costs of work crews, but not including the costs of the materials.
In the past, experiments have been performed in which the goal was to prove the efficacy of pumping cable into a reel of coiled tubing. These experiments involved pumping water or other suitable fluid into a reel of coiled tubing to generate a flow of sufficient velocity through the tubing to drag the cable through.
The problem with pumping a cable in occurs at the pressure boundary. Movement of the cable through the pressure boundary is resisted primarily by the pressure differential between the pressurized reel and outside ambient pressure as well as friction forces at the stuffing box or seal where the cable enters the pressurized reel. Therefore, the total force to inject the cable equals the cross-sectional area of the cable times that pressure trying to push it back out plus the force necessary to overcome the friction resistance of the seal.
These experiments were partly successful in proving that cable could, indeed, be pumped into a reel but experienced several shortcomings. The major shortcoming of this method was that cable could only be pumped into the reel a relatively short distance. After a relatively short distance, the force necessary to assist the cable into the reel became too great. This is a major drawback since, to be worthwhile, the cable must be installed into the entire length of coiled tubing, often 10-20,000 feet.
On the other hand, one major success of these experiments was the proof that the cable could be removed from a reel of tubing using the pumping technique. This proof, however, did not solve the problem of installing cable into coiled tubing while it is still on the reel.
One proposed solution to the cable installation problem involved placing a set of rollers within the pressure chamber that included the coiled tubing. This proposed solution proved difficult to implement and caused damage to the cable due to the small area of contact of the rollers with the cable. This technique also was incapable of sensing when the cable stopped moving into the coiled tubing and therefore created kinking of the cable.
The recognition of the drawback of the rollers inside the pressure led to another possible solution that involved putting the entire spool of cable inside a pressure chamber. The pressure chamber was to be made from refuse pipeline of about 4 feet in diameter. This arrangement eliminated the difficulty of drawing the cable into the differential pressure and the seal since there was no need for a seal. The problems with this apparatus were the cost and controllability of cable insertion. Difficulty was encountered in aligning the cable with the end of the coiled tubing and this technique proved unrealistically expensive to implement.
Thus, the solutions that have heretofore been proposed to solve the dilemma of installing cable into coiled tubing while it remains on the reel have all been less than satisfactory for one reason or another. Some such techniques have failed to install cable into sufficient length of tubing, some have offered significant promise but have proved impractical in application, while still others have caused damage to the cable or have been as expensive to implement as the old techniques that they were intended to replace. Therefore, there remains a need for a method and apparatus for the installation of cable into coiled tubing that is practical, effective, and yet cost effective.